Conventionally, metal substrates of an aluminum alloy or the like have been widely used as substrates of magnetic recording media. These metal substrates are processed to form circular grooves on their surfaces, which grooves are referred to as a texture. The texturing on the substrate surface prevents the magnetic head from contacting the recording medium to prevent wear, which can happen when the magnetic head flies over the magnetic recording medium during seeking, and to orient the in-plane magnetization direction in the circumferential direction, which is the recording direction. High-density magnetic recording can be formed with an “anisotropic medium,” where magnetic anisotropy is formed in the medium plane (in-plane magnetic anisotropy). The output characteristics of the recording medium correspond to the product of the magnetic layer thickness and the remanent magnetization (hereinafter described as thickness remanent magnetization product (Mrt)).
With a medium having in-plane magnetic anisotropy, where the output characteristic corresponding to the thickness remanent magnetization product (Mrt) in the circumferential direction is set to constant, the remanent magnetization (Mrt) in the circumferential direction is higher than in the case of an in-plane isotropic medium. Thus, the magnetic layer can be made thinner and medium noise caused by the magnetic layer can be reduced. Since the S/N ratio (signal-to-noise ratio) and so on can be improved, a high-density recording medium can be obtained. Note that the ratio between the thickness remanent magnetization product in the circumferential direction (Mrt(Cir)) and the thickness remanent magnetization product in the radial direction (Mrt(Rad)), namely (O.R.-Mrt=Mrt(Cir)/Mrt(Rad)) can be used as an indicator of the in-plane magnetic anisotropy. The larger O.R.-Mrt, the larger the in-plane magnetic anisotropy.
At present, so-called “isotropic media” are the mainstream magnetic recording media that use a nonmetallic substrate, such as a glass substrate or a ceramic substrate. But anisotropic magnetic recording media that use a nonmetallic substrate are being contemplated. Compared with an anisotropic magnetic recording medium that uses a metallic substrate, an anisotropic magnetic recording medium that uses a nonmetallic substrate has better shock resistance for use in mobile equipment, and higher smoothness for realizing high-speed rotation for a high-speed server, i.e., higher rigidity. On the other hand, a magnetic recording medium that uses a nonmetallic substrate, even if texturing is carried out on the surface of the nonmetallic substrate, it is difficult to form a magnetic layer having a sufficient in-plane magnetic anisotropy. This is due to the thin film crystal orientation and the grain diameter being different and the thermal expansion of the substrate being small compared with that of the metallic substrate.
Obtaining a magnetic recording medium that uses a textured nonmetallic substrate is difficult as described above, but proposals to overcome the noted problem have been made. For instance, Japanese Patent Application Laid-open No. 2001-331934 discloses that by sputtering in oxygen or nitrogen environment to deposit an orientation adjusting film that adjusts the orientation of a film immediately thereupon having Ni—P as a principal component onto a nonmetallic substrate of glass or the like that has been subjected to texturing, then oxidizing the surface of the orientation adjusting film, and then depositing a nonmagnetic underlayer and a magnetic film on the orientation adjusting film that has been subjected to oxidation treatment without removing the medium substrate from the film-depositing apparatus, magnetic anisotropy can be created in the medium plane to obtain a high-density recording medium using a manufacturing process that is simpler.
Recently, however, the recording density of magnetic recording media has continued to increase at an ever greater rate, and hence magnetic recording media enabling the realization of yet higher density magnetic recording are desired. If the magnetic recording density is increased with an in-plane magnetic recording medium, the area of the medium per recorded bit becomes smaller, and hence the playback output drops, and playback becomes difficult. Although the playback problem can be improved by using a high-sensitivity head (which increases the playback output), the medium noise is amplified at the same time to make reading of recorded information difficult. To make an in-plane magnetic recording medium have high recording density, it is essential to reduce the medium noise. Moreover, to win out over the diamagnetic field from magnetization at bit boundaries and maintain the magnetization in the recording direction, it is necessary to increase the remanent coercivity (Hcr), and also reduce the thickness remanent magnetization product to reduce the diamagnetic field. The “remanent coercivity (Hcr)” refers to the coercivity in the remanent curve obtained through measurement of magnetic relaxation (remanence). If medium noise is reduced by reducing the thickness of the magnetic layer and decreasing the crystal grain size of the magnetic layer, the remanent coercivity drops dramatically and the playback output drops dramatically, and also the thermal stability of the recording magnetization state becomes poor so that loss of recorded information becomes problematic. On the other hand, as described above, by increasing the in-plane magnetic anisotropy, the medium noise caused by the magnetic layer can be reduced to improve the S/N ratio. Moreover, a high remanent coercivity also becomes necessary as one factor for maintaining the playback output, i.e., improving the thermal stability.
There is a need for a high-density magnetic recording medium that uses a nonmetallic substrate having both the high in-plane magnetic anisotropy and the high remanent coercivity, while reducing medium noise to improve the S/N ratio. The present invention addresses this need.